This invention relates to a system for manufacturing springs of any prescribed free length, and to a method of manufacturing such springs
When manufacturing springs using a system of the aforementioned type in the prior art, the general practice is to manufacture the springs upon setting parameters related to spring manufacture. Even when various parameter data concerning spring manufacture have been set, however, springs having the same free length do not always result, so that there is usually a certain degree of variance from one spring to another.
The reasons for the above primarily are a change in the wire material, wire characteristics such as a non-uniformity in the cross-sectional shape (diameter, etc.) thereof, and a change in the environment, such as a change in temperature, at the time of manufacture. In particular, a change in wire characteristics owing to a difference among wire lots is a matter of course, but there are also slight variations among the wires in one and the same lot.
When there is a change in the wire characteristics or a change in temperature, this is ultimately accompanied by a change in the elasticity of the wire. By way of example, even when a spring is manufactured by moving a swivel shaft in the axial direction while forcibly winding a wire on the swivel shaft, the wire attempts to return to its original shape to a slight degree owing to its elasticity. As a result, a spring having the pitch and number of turns that prevailed at winding cannot be manufactured. Accordingly, when it is considered that the elasticity of a wire is constantly changing when a spring is being manufactured, it can be understood how difficult it is to manufacture springs having a fixed, free length.
Let springs which fall within allowable limits with regard to their desired free length be defined as "acceptable" or "non-defective" springs. In spring manufacture, what is important is to develop an expedient for raising the acceptance rate or yield, namely the ratio of the number of non-defective springs to the total number of springs manufactured In the prior art, however, there are many aspects in which reliance is placed upon the intuition or skill of the worker in an attempt to achieve this, and a firm set of relevant techniques has not yet been established.